1. Technical Field of the Invention
The present invention relates to circuit and method for writing to a memory disk, and particularly to a circuit and method for driving the write head of a disk drive device.
2. Background of the Invention
Most computer systems include one or more associated disk drives, which may be built into or external to the computer system. Typically, disk drives have at least one rotating magnetic medium and associated head mechanisms that are carried adjacent the magnetic material. The heads are radially positionable to selectively write information to, or read information from, precise positions on the disk medium. Such disk drives may be, for example, hard disk drives, floppy drives, or the like.
Data is written to the associated data disk by applying a series of signals to a write head according to the digital information to be stored on the magnetic disk media. The write head has a coil and one or more associated pole pieces that are located in close proximity to the disk media. As signals cause the magnetic flux to change in the head, the magnetic domains of the magnetic media of the disk are aligned in predetermined directions for subsequent read operations. Typically, a small space of unaligned magnetic media separates each magnetic domain transition to enable successive transitions on the magnetic media to be distinguished from each other.
Since the disk is moving relative to the head, it can be seen that if the small space separating the magnetic domain transitions is not sufficiently wide, difficulty may be encountered in distinguishing successive magnetic transitions. This may result in errors in reading the data contained on the disk, which is, of course, undesirable.
Meanwhile, as computers are becoming faster, it is becoming increasingly important to increase the speed at which data can be written to and read from the disk media. However, since the data signals are in the form of square wave transitions, if the rise time of the leading edges of the square waves is large, the small space between magnetic media transitions also becomes large, which reduces the effective rate at which data can be accurately written and read. Since the write head assembly includes at least one coil, forcing the current to rise rapidly, or to reverse flux directions within the write head is difficult.
In the past, data writing circuits used to supply such write signals to the heads included preamplifier circuits to drive the current through selected legs of an xe2x80x9cH-bridgexe2x80x9d circuit, which is capable of allowing relatively fast current reversals for accurate data reproduction.
An example of a typical H-bridge write head data driving circuit 10, according to the prior art, is shown in FIG. 1. The circuit 10 includes four MOS transistors, 12-15 connected between a Vcc voltage 11 and ground reference 17. A coil 19, used, for example, to supply data pulses for writing to a disk drive media is integrated into the write head mechanism. The coil 19 is connected between the center legs of the H-bridge, as shown.
It can ben seen that, depending on the gate biases applied to the respective transistors 12-15, the current flows through the coil 19 in one direction or another. That is, one current flow path includes the transistor 14, coil 19 from right to left, and transistor 13. The other current flow path includes transistor 12, the coil 19 from left to right, and the transistor 15.
In the H-bridge circuit 10, the transistor 12 and 14 serve as switching transistors, which are controlled by the out-of-phase signals on a pair of respective input lines 28 and 29. The transistors 13 and 15 serve as current controlling transistors, which are controlled by the out-of-phase signals on the respective input lines 29 and 28 in a manner opposite from the connections to the switching transistors 12 and 14, via respective control transistors 31 and 32. The magnitude of the current through the transistors 13 and 15 is controlled by a transistor 21, with which the transistors 13 and 15 form respective current mirrors, when connected via respective transmission gates 24 and 25. The transmission gates 24 and 25 are controlled by the signals on the respective input lines 29 and 28, in the same manner as the associated transistors 31 and 32. A reference current source 26 supplies the reference current to the transistor 21, which is mirrored by currents in respective transistors 13 and 15, as described above.
Thus, the data drive signals supplied to the head mechanism associated with the circuit 10 may be controlled by applying appropriate signals to the input lines 28 and 29. However, as mentioned, as data rates increase, the rates at which the heads can accurately write the data to the magnetic media is limited by the speed at which the flux in the coil 19 (and its associated components) can be reversed. The maximum data rate is thus limited to the maximum physical flux reversal rate of the driver circuitry.
What is needed, therefore, is a method and circuit for driving an inductive load of the type used in conjunction with a write head of a disk drive with a signal that enables a maximum flux reversal rate in the driver coil.
The present invention overcomes the shortcomings in prior systems and thereby satisfies a significant need for a driver circuit for the write head of a disk storage device. The driver circuit provides a current to the write head so that current flows through the write head in one direction or the other. The driver circuit includes a pair of identical sub-circuits, each sub-circuit being connected to a distinct terminal of the write head. Each driver sub-circuit forms a leg of an H-bridge driver circuit.
Each driver sub-circuit includes a first pull-up and/or switching device having a first terminal connected to a high voltage reference and a second terminal coupled to the corresponding write head terminal. The driver sub-circuit further includes a current sink circuit capable of sinking current from the corresponding terminal of the write head. The driver circuit further includes a control circuit connected to the first pull-up device and the current sink circuit of each driver sub-circuit, for controlling current flow through the write head so as to write data on a corresponding magnetic storage disk.
In general terms, the control circuit activates the first pull-up device of one driver sub-circuit so as to provide a current to one terminal of the write head, while activating the current sink circuit of the other driver sub-circuit in order to sink the provided current from the other terminal of the write head. In this way, the control circuit is capable of passing a predetermined current level through the write head in either direction as desired to write data on the magnetic storage disk.
As stated above, it is desirous for H-bridge driver circuits to cause the current flowing through the write head to relatively quickly switch directions. In order to lessen the transition time between the desired steady state current level flowing through the write head in one direction and desired steady state current level flowing therethrough in the opposite direction, each driver sub-circuit further includes a bootstrap circuit for temporarily pulling the voltage appearing at the corresponding write head terminal below the low voltage reference. The bootstrap circuit causes the voltage appearing across the write head to relatively quickly become substantially large, thereby causing a relatively large amount of current to pass through the write head. Consequently, the current flowing through the write head quickly transitions from one steady state current level to the other.
The bootstrap current sink circuit is activated by the control circuit for a first predetermined period of time corresponding to the current level in the write head approximately reaching or relatively slightly surpassing (i.e., overshooting) the intended destination steady state current level. The control circuit thereupon deactivates the bootstrap circuit so as to limit the amount of overshoot of current flowing through the write head. Because there may be an appreciable amount of ringing at the corresponding write head terminal despite the deactivation of the bootstrap circuit, each driver sub-circuit further includes a second pull-up device coupled to the corresponding write head terminal. The second pull-up devices are utilized to relatively quickly force the write head current to the destination steady state current level.
In particular, substantially immediately after the bootstrap circuit is deactivated by the control circuit, the second pull-up device associated with the same write head terminal is temporarily activated by the control circuit for a second predetermined period of time. The activated pull-up device pulls the voltage level appearing at the corresponding write head terminal towards the voltage level appearing on the other write head terminal. At the end of the second predetermined period of time, the voltage across the write head is near zero volts, thereby yielding a substantially constant current through the write head at the desired steady state current level. The current flowing through the write head is maintained at approximately the desired steady state current level by the current sink circuit following deactivation of the activated second pull-up device. In this way, the time associated with reversing current through the write head is minimized without experiencing substantial current overshoot or undershoot relative to the desired destination steady state current level.
According to a preferred embodiment of the present invention, each driver sub-circuit includes a protection transistor connected in series between the first and second pull-up devices and the corresponding terminal of the write head. For each protection transistor, the control terminal is tied to a fixed voltage so that a finite on resistance is created between the two current conducting terminals of the protection transistor. The voltage swings or voltage differentials appearing across the terminals of each of the pull-up devices are correspondingly reduced. The reduced voltage swing appearing across the terminals of each of the pull-up devices allows for the pull-up devices to be implemented with low voltage transistors having lower breakdown voltages, which results in increased circuit performance.